The invention relates to a flux composition for use in soft soldering.
Soft solders are used in many industries, but particularly in the electronics industry where they are used for soldering printed circuit boards. For this and all other applications the solder must be able to wet the substrate, if the solder is to form a good bond. Thus a flux will be employed to improve the ability to wet the substrate. One of the properties which a flux should possess is the ability to remove surface oxide film from the substrate and thus the constituents of a flux are often both active and corrosive.
In the electronics industry, however, it is desirable to use fluxes of fairly low activity and which are non-corrosive in that they will not result in corrosion of the soldered joint over a period of time even if it becomes damp. Corrosion is most often caused by corrosive flux residues being left on the substrate. Thus any residue left should be non-corrosive and also leave little residue as possible.
Fluxes for use with printed circuit boards have been formulated using natural rosin as the basic constituent, usually together with other ingredients such as flux-activating agents. These fluxes have been sold either in the form of cored wire or as a solution where the rosin and other solid components are dissolved in a suitable solvent. However, such rosin fluxes have tended to leave appreciable quantities of residue which, although generally non-corrosive, are undesirable. The residues may be unsightly and their mere presence on a circuit board may cause problems. For example, if automatic circuit test equipment is used the contact pins may become clogged with flux residues. Some residues may be removed by washing the circuit boards with chlorofluorocarbons (CFCs), but this necessitates a further processing step, thus increasing cost and also CFCs are known to be bad for the environment.
Liquid fluxes containing active ingredients in a solvent are preferred for use with printed circuit boards because of their ease of application, for example, by immersion, foaming or spraying.
It is therefore desirable to produce liquid flux systems which contain a low solids content and which, after completion of the soldering process leave little residue.
British Patent No. 1550648 aims to reduce the unpleasant fumes associated with traditional fluxes containing rosin. It describes flux compositions comprising as an essential constituent at least one neutral ester of a polyhydric alcohol having a molecular weight of at least 300, and at least one additional constituent selected from organic acids, flux-activating agents and flux residue-hardening agents. However, in many of the examples a dimerised or other rosin compound is employed as the flux residue hardening agent and thus these compositions still leave appreciable residues.
Furthermore, the usefulness of a flux can vary considerably depending on the conditions of use and the sort of application. In some cases a flux system may work well with one circuit board surface but unsatisfactorily with another. One possible reason for this is that the flux system has to interact with so many different parts. It interacts not only with the printed circuit board surface but also with the resist coating, component leads and components, and also with the metallic surfaces of conductors, tracks, plated holes and pads. Thus, it is not surprising that the effectiveness of fluxes can vary depending on the particular nature of the soldering environment and the materials used. There remains a demand for further and improved fluxes to be developed.
We have now developed a composition suitable for use as a liquid soft soldering flux and which has the advantage of producing a very small amount of residue.